The paper presents the numerically evaluated dependence of the stress intensity factor (SIF) on crack velocity (K_I- a˙ dependence) in Homalite-100 specimens subjected to pulse loading. Experiments on crack propagation (Ravi-Chandar and Knauss in Int J Fract 25:247–262, 1984. https://doi.org/10.1007/BF00963460; Int J Fract 26:65–80, 1984. https://doi.org/10.1007/BF01152313; Int J Fract 26:141–154, 1984. https://doi.org/10.1007/BF01157550; Int J Fract 26:189–200, 1984. https://doi.org/10.1007/bf01140627) were simulated using the finite element method and incubation time fracture criterion. According to (Ravi-Chandar and Knauss in Int J Fract 26:141–154, 1984. https://doi.org/10.1007/BF01157550), experimental data on the SIF–crack velocity dependence exhibit unstable behavior, i.e. considerable scattering of the SIF values: a broad range of SIF values corresponds to a single crack velocity. This way, the conventional approach based on a K_I- a˙ dependence being a material property is not applicable in this case. Such a phenomenon is also observed in a numerically obtained K_I- a˙ dependence, meaning that the developed approach makes it possible to evade the known ambiguity of the K_I- a˙ relation to predict the crack propagation.